1. Field of the Invention
The present invention concerns a novel antenna structure, that has a quasi-hemispherical radiation pattern, and is capable of having a relatively wide passband, so as to make it possible, for example, to define two neighboring sub-bands therein.
This type of antenna can be applied, for example, in the context of satellite communications between fixed users and aeronautical, naval and land-based moving bodies. In this field, several satellite communications systems have been undergoing development in L band (for example INMARSAT, MSAT, PROSAT, NAVSTAR, G.P.S. etc.).
The first three systems referred to correspond to links with geostationary satellites. In these systems, the specifications of the antennas designed to fit out the moving bodies make it necessary for these antennas to have a radiation pattern with a quasi-hemispherical coverage, owing to very different incidences and or major variations in incidence of the received or transmitted signals.
Furthermore, the polarization of the antennas should be circular with an ellipticity of more than 5 dB (20 dB isolation) and special attention has to be paid to combating multiple-path phenomena for air and land-based moving bodies. This latter specification, moreover, makes it necessary for the preponderant component of the electrical field to be vertical for low elevations.
As for antennas which can be used at the reception of signals by transiting satellites used in systems of the U.S. NAVSTAR type, the specifications lay down that they should be operational in a passband of about 10% or in two neighboring sub-bands.
2. Description of the Prior Art
In the present state of the art, the only antenna structure compatible with this type of specification (essentially a quasi-hemispherical radiation pattern and circular polarization) is the resonant quadrifilar helix.
This type of known antenna, as shown in FIGS. 11A, 11B, is formed by two bifilar helices 111, 112, positioned orthogonally and excited in phase quadrature.
The exemplary structure shown in FIGS. 11A, 11B is cited in the work "UHF Satellite Array Nulls Adjacent Signals", Microwave & R.F., March 1984.
The antenna is the resonant quadrifilar helix with wires 111A, 111B; 112A, 112B short-circuited at their non-excited end 113. The passband is in the range of 10% with a 140% aperture at -3 DB for a wire length equal to .lambda..sub.o /2 and a helical winding on a half turn. This type of antenna must not be mistaken for certain helical antennas of the type disclosed, for example, in the patent document U.S. Pat. No. 4,148,030 (FOLDES), the purpose of which is to provide highly directional (not quasi-hemispherical as in the invention) and high-gain axial radiation patterns. Their operation is of the travelling wave type, and they do not work in resonant mode. Moreover, these known antennas have a different structure. They have, in particular, a length that is several times the operating wavelength .lambda. of the antenna. Besides, each helical wire is made of a plurality of resonating dipoles, to work at a specific frequency.
There is also another known embodiment of a quadrifilar helical antenna, used in INMARSAT STANDARD-C satellite communications between moving bodies, where the antenna must work accurately in two sub-bands (1530-1545 MHz) and (1631.5-1646.5 MHz) corresponding respectively to reception and transmission (K. M. KEEN "Developing a Standard-C Antenna", M.S.N. Communications Technology, June 1988).
In this known embodiment, the antenna is a resonant quadrifilar helix with printed wires open at their non-excited end.
Although the resonant quadrifilar antennas meet the requisite specifications, they have a number of drawbacks.
The main problems posed by this known type of structure relate to the constraints of matching the impedance values of the antenna with those of the coaxial feed lines while, at the same time, achieving adequate excitation of the orthogonal bifilar helices.
In the narrow band systems, the feed/matching module may be positioned externally to the antenna, around the working frequency. But, when the antenna has to work in a wideband, as discussed herein, a feed/matching antenna internal to the antenna structure is generally used. The most common one is the so-called "balun" (sometimes also called a "symmetrizer") system or its variant, the "folded balun" with dissymmetrical input and symmetrical output.
An assembly such as this is shown in FIG. 11 where, taking account of the excitation and symmetry of structure of the antenna, the two orthogonal helices 111 and 112 have the same input impedance. Each bifilar helix 111A, 111B; 112A, 112B is fed by a folded balun type of coaxial symmetrizer. The two bifilars are then excited in phase quadrature by means of a hybrid coupler 115 (90.degree., -3 DB). Each coaxial (dissymmetrical) input therefore sees, in parallel, the impedance of the bifilar helix and a length adapter in the neighborhood of .lambda./4.
The symmetrizer/adapter assembly used in this type of antenna is made, for example, by means of a coaxial section with a length .lambda./4, the core and sheath of which form a dipole. To circumvent the problems due to the radiation from the sheath, the dipole may be enclosed between the core and an additional coaxial sheath (bazooka system) so as to prevent the flow of a current on the sheath of the coaxial line.
However, this type of assembly has the drawback of forming a sort of passband filter with a band that is still too narrow.
More complex systems were then conceived of, using a line compensated for by means of a solid conductor or, again, a dead coaxial cable forming a trap circuit (see C. C. Kilgus, "Resonant Quadrifilar Helix", Microwave Journal, December 1970).
In any case, a matching device must be added between the hybrid coupler and the "baluns" to match the antenna. This emerges clearly, in particular from the Smith pattern in FIG. 12 where it is clearly seen that, for two embodiments, the operating windows 121, 122 are essentially outside the matching zone 123.
Now, the use of matching devices introduces losses and often restricts the band of use of the antenna. Furthermore, in these exemplary embodiments, and certainly for reasons related to the space factor, the "folded balun" is placed in the very body of the antenna excited at its upper end. This then produces a disturbance by diffraction of the radiation patterns, particularly at the high frequencies.
It is an object of the invention to overcome these drawbacks.
More precisely, the invention provides a new antenna structure with an almost hemispherical radiation pattern and with circular polarization, notably (but not exclusively) in L Band.
Another aim of the invention is to provide a structure such as this, that avoids the need for introducing complex matching means between the antenna and its excitation.
It is also an aim of the invention to provide an antenna with a widening of the passband, or a dual frequency operation, notably either in a passband.apprxeq.10% or in two neighboring passbands.
An additional object of the invention is to give a low-cost antenna with energy consumption compatible with the constraints of systems on board land-based, sea, air or space craft.
These aims, as well as others that shall appear here below, are achieved according to the invention by means of a resonant helical antenna with quasi-hemispherical radiation, of the type having a quadrifilar helix, formed by two bifilar helices arranged orthogonally and excited in phase quadrature, said antenna having at least one second quadrifilar helix that is coaxial and electromagnetically coupled with said first quadrifilar helix, each of said quadrifilar helices being wound around a distinct cylinder, with a constant radius.
The overlapping of these two resonant quadrifilar helices makes it possible to obtain a quasi-hemispherical radiation pattern over a wide frequency band, or over two neighboring frequency bands, depending on the settings chosen for their electromagnetic coupling.
Advantageously, the length of the wires is smaller than the wavelength .lambda. of operation of said antenna, and is preferably between .lambda./2 and .lambda., so as to obtain the desired hemispherical pattern, with operation in standing wave mode.
According to a preferred characteristic of the invention, the wires of said second quadrifilar helix are in a position of precise or near radial overlapping, with the wires of said first quadrifilar helix.
According to another characteristic of the invention, said coupled quadrifilar helices are connected in parallel to a common feeder. Advantageously, said common feeder includes, firstly, a coupler element for the excitation, in phase quadrature, of the two orthogonal bifilar helices of each quadrifilar helix and, secondly, a symmetrizer element for the feeding, in phase opposition, of each of the wires of the bifilar helices.
Preferably, the wires of at least one of the two quadrifilar helices are open or short-circuited at their non-excited end.
Advantageously, at least one of the quadrifilar helices is made by means of printed circuit technology on dielectric support.
According to an advantageous characteristic of the invention, the coupling of said quadrifilar helices is controlled through at least one of the following means:
checking of the radial divergence of overlapping of said quadrifilar helices; PA1 checking of the angular offset between said quadrifilar helices; PA1 checking of the helix pitch of each of said helices, in particular so as to match the impedance presented by each wire.
According to a first embodiment, said coupling of said quadrifilar helices is done so as to obtain a radiation of the antenna in a single wide passband.
According to a second embodiment, said coupling of said quadrifilar helices is done so as to obtain a radiation of the antenna in at least two passbands that are apart.
It is clear that, through the invention, the checking of the coupling can be optimized, without lowering any of the other characteristics of the antenna, and in particular the circular polarization and the radiation pattern.